Bacillus thuringiensis (commonly known as B.t.) is a gram-positive soil bacterium that often produces cellular inclusions during sporulation which are specifically toxic to certain orders and species of insects. Many different strains of B.t. have been shown to produce these inclusions of insecticidal crystal protein (ICP). Compositions including B.t. strains which produce insecticidal proteins have been commercially available and used as environmentally acceptable insecticides because they are quite toxic to the specific target insect, but are harmless to plants and other non-targeted organisms.
B.t. ICP toxins are active in the insect only after ingestion. After ingestion by an insect, the alkaline pH and proteolytic enzymes in the mid-gut solubilize the crystal allowing the release of the toxic components. These toxic components disrupt the mid-gut cells resulting in cessation of feeding and, eventually, death of the insect. B.t. has proven to be an effective and environmentally safe insecticide in dealing with various insect pests.
A number of genes encoding crystal proteins have been cloned from many strains of B.t. A good overview is set forth in H. Hofte and H. R. Whiteley, Microbiol. Rev., 53, pp. 242-255 (1989), hereinafter "Hofte and Whiteley (1989)." This reference provides a good overview of the genes and proteins obtained from B.t. and their uses, adopts a nomenclature and classification scheme for B.t. genes and proteins, and has an extensive bibliography.
The nucleotide sequences of ICP genes responsible for a given crystal phenotype and active against the same insect order are generally more related, i.e., more homologous, than are the nucleotide sequences of B.t. genes encoding delta-endotoxin proteins active against different orders of insects. Hofte and Whiteley (1989) defines an ordered classification of genes encoding B.t. delta-endotoxin proteins based on homology of delta-endotoxin amino acid sequences, as well as similarities in insecticidal activity; a subranking has also been established based upon further refinement of sequence relationship. As noted by Hofte and Whiteley (1989), the majority of insecticidal B.t. strains are active against insects of the order Lepidoptera, i.e., caterpillar insects. Insecticidal crystal proteins specifically active against Lepidoptera have been designated CryI proteins. These ICPs are encoded by cryI genes. Other B.t. strains produce different classes of crystal proteins, e.g., CryII proteins are active against lepidopteran and (for CryIIA) dipteran insects; CryIII proteins are insecticidal to insects of the order Coleoptera, i.e., beetles; and CryIV proteins are active against insects of the order Diptera, i.e., flies and mosquitoes. A compilation of the amino acid identities for several CryI proteins as well as CryII, CryIII and CryIV proteins has been determined in Hodgman and Ellar, J. DNA Sequencing and Mapping, 1, pp. 97-106 (1990).
The CryI family of ICPs contains the largest number of known toxin genes derived from B.t., as evidenced by the survey in Hofte and Whiteley (1989) and by subsequent reports of CryI-type ICPs.
Schnepf et al., J. Biol. Chem., 260, pp. 6264-6272 (1985), reported the complete nucleotide sequence for a toxin gene from B.t. kurstaki HD-1. This gene was subsequently classified as cryIA(a) by Hofte and Whiteley (1989). The published open reading frame extends 1176 amino acids and encodes a protein with a calculated molecular mass of 133,500 Daltons (Da). Another gene, also classified as cryIA(a), was isolated from B.t. subsp. kurstaki HD-1 Dipel.RTM. by Shibano et al., Gene 34, pp. 243-251 (1985). As detailed in Table 2 of Hofte and Whiteley (1989), this gene is highly related, especially in the N terminal moiety, to cryIA(a) reported by Schnepf et al. (1985). CryIA(a) protein is broadly active against Lepidoptera; Hofte and Whiteley (1989) reports that four of five tested lepidopteran insects were sensitive to this toxin.
Other ICP genes subsequently identified as cryIA(a) that are greater than 99% identical to the holotype cryIA(a) gene have been identified in B. thuringiensis subspecies aizawai, (Shimizu et al., Agric. Biol. Chem., 52, pp. 1565-1573 (1988)), subspecies kurstaki, (Kondo et al., Agric. Biol. Chem., 51, pp. 455-463 (1987)), and subspecies entomocidus (Masson et al., Nucleic Acids Res. 17, p. 446 (1989)). The cryI-type nucleotide sequence disclosed in European Patent Application Publication No. 0 367 474, published May 9, 1990, of Mycogen Corporation, reveals a DNA sequence related to the cryIA(a) gene and its encoded protein that is 92% positionally identical to the holotype CryIA(a) ICP.
Wabiko et al., DNA, 5, pp. 305-314 (1986), describe the DNA sequence of an insecticidal toxin gene from B.t. subsp. berliner 1715, subsequently classified as cryIA(b) by Hofte and Whiteley (1989). The molecular mass of the protein encoded is 130,615 Da and sequential deletions indicate that the NH.sub.2 -terminal 612 amino acid polypeptide is toxic to lepidopteran insects. Hofte et al., Eur. J. Biochem., 161, pp. 273-280 (1986), describe the cloning and nucleotide sequencing of a variant crystal protein gene from B.t. subsp. berliner 1715, subsequently also classified as cryIA(b). The cloned gene produces an approximately 130,000 Da protein which coincides with the mass of the major protein observed in the strain. The gene has an open reading frame of 3465 bases which would encode a protein 1155 amino acids in length having a mass of 130,533 Da. Similarities of this sequence to the previously reported sequences for the cloned crystal genes from B.t. kurstaki HD-1, B. t. kurstaki HD-73 and B.t. sotto are discussed in the Hofte et al. (1986) paper. Data identifying a minimal toxic fragment required for insecticidal activity are also presented. The cryIA(b) gene discussed in Hofte et al. (1986) differs in its deduced amino acid sequence by only two amino acids from the CryIA(b) protein reported by Wabiko et al.
Other cryIA(b) genes have been disclosed in Geiser et al., Gene, 48, pp. 109-118 (1986), Hefford et al., J. Biotechnol., 6, pp. 307-322 (1987), Oeda et al., Gene, 53, pp. 113-119 (1987), Kondo et al., supra, Fischhoff et al., Bio/Technology, 5, pp. 807-813 (1987) and Haider and Ellar, Nucl. Acids Res., 16, p. 10927 (1988). Each of these six CryIA(b) ICPs is greater than 99% positionally identical to the holotype CryIA(b) toxin.
Adang et al., Gene, 36, pp. 289-300 (1985), report the cloning and complete nucleotide sequence of a crystal protein gene harbored on the 75 kilobase (kb) plasmid of strain B.t. subsp. kurstaki HD-73. The restriction map in the article identified this gene as holotype cryIA(c) under the current classification system of Hofte and Whiteley (1989). The complete sequence of the gene, spanning 3537 nucleotide base pairs (bp), coding for 1178 amino acids and potentially encoding a protein of 133,330 Da, is shown in the article. Toxicity data against Manduca sexta for the protein made by the cryIA(c) gene are also presented. CryIA(c) toxins have been isolated from several strains of B.t. subsp. kenyae that are highly related to the above-noted CryIA(c) toxin from B.t. subsp. kurstaki (greater than 99% positionally identical in deduced amino acid sequence) but whose protein products, although broadly active against lepidopteran insects, nonetheless show quantitatively different toxicities for individual insect species (Von Tersch et al., Appl. Environ. Microbiol., 57, pp. 349-358 (1991)).
Brizzard et al., Nucleic Acids Res., 16, pp. 2723-2724 (1988), describe the nucleotide sequence of crystal protein gene cryA4 (subsequently classified as cryIB by Hofte and Whiteley (1989)) isolated from B.t. subsp. thuringiensis HD-2. Hofte and Whiteley (1989) report an insecticidal specificity of CryIB toxin for Pieris brassicae.
Honee et al., Nucleic Acids Res., 16, p. 6240 (1988), describe the complete DNA sequence for the BTVI crystal protein gene isolated from B.t. subsp. entomocidus 60.5 (holotype cryIC by Hofte and Whiteley (1989)). This protein is reported to exhibit enhanced insecticidal activities against Spodoptera species.
Sanchis et al., Mol. Microbiol., 3, pp. 229-238 (1989) report the nucleotide sequence for the N-terminal coding region (2470 nucleotides) and 5' flanking region of a gene from B.t. subsp. aizawai 7.29 now classified as the cryIC gene under the classification system of Hofte and Whiteley (1989). Sanchis et al. disclose similar information about the cryIC gene in European Patent Application Publication No. 0 295 156, published Dec. 14, 1988. The open reading frame encodes a truncated polypeptide 824 amino acids long with a calculated mass of 92,906 Da.
A gene isolated from B.t. subspecies aizawai and now classified as holotype cryID under the Hofte and Whiteley (1989) system is disclosed in European Patent Application Publication No. 0 358 557, published Mar. 14, 1990 of Plant Genetic Systems, N. V. Hofte and Whiteley (1989) report selective lepidopteran toxicity against Manduca sexta for the CryID protein, the CryID toxin being largely inactive against other lepidopteran insects tested.
The holotype cryIE gene, found in a B.t. subspecies darmstadiensis strain, is disclosed in European Patent Application Publication No. 0 358 557, supra. A highly related cryIE gene from B.t. subsp. kenyae is disclosed by Visser et al., J. Bacteriol., 172, pp. 6783-6788 (1990).
Visser, Mol. Gen. Genet., 212, pp. 219-224 (1988) report the isolation and analysis of five toxin genes belonging to four different gene families from B.t. entomocidus 60.5, one of which is reported by Honee et al. (1988), supra. Two of these genes, BTIV and BTVIII, are cryIA(a)-type genes according to the Hofte and Whiteley (1989) classification scheme. The BTVI gene, also reported by Honee et al. (1988) supra, is a cryIC gene according to the Hofte and Whiteley (1989) classification scheme. The authors state that the restriction map for another gene, designated BTV, closely resembles that identified for the cryID gene isolated from B.t. strain HD68 subsp. aizawai, and disclosed in European Patent Application Publication No. 0 358 557, supra. A fifth gene, designated BTVII, is also identified and its restriction map differs significantly from the other four genes described. Toxicity data against several lepidopteran insects, S. exigua, S. littoralis, H. virescens and P. brassicae, are presented for each of the isolates. The BTV gene product was inactive against all insects tested. The BTVI protein is highly active against Spodoptera larvae, and the BTVII protein is toxic to P. brassicae.
Additional genes within the cryI family have been reported in the literature. A gene found in B.t. subsp. aizawai and described as cryIF is disclosed by Chambers et al. in J. Bacteriol., 173, pp. 3966-3976 (1991) and in PCT International Publication No. WO91/16434, published Oct. 31, 1991. A gene described as cryIG from B.t. subsp. galleria is disclosed by Smulevitch et al., FEBS Lett., 293, pp. 25-28 (1991). A gene that is highly related to the cryIG gene has been isolated from B.t. DSIR 517 by Gleave et al., J. Gen. Microbiol., 138, pp. 55-62 (1992).
A novel gene related to cryI-type genes is disclosed in PCT International Publication No. WO 90/13651, published Nov. 15, 1990, of Imperial Chemical Industries PLC. This gene encodes an 81 kDa polypeptide (Cry pJH11) that is broadly insecticidal and more distantly related to the family of cryI sequences than are most other reported cryI-type sequences. Four cryI-type sequences are disclosed in European Patent Application Publication No. 0 405 810, published Jan. 2, 1991, of Mycogen Corporation. Inspection of the cryI-type sequences revealed that one of the disclosed genes (cry 81IB2) belongs to the cryIC class, one (cry 81IB) belongs to the cryID class, and one (cry 81IA) belongs to the cryIF class. The fourth disclosed cryI sequence (cry 81IA2) appears to belong to a new class. Two cryI sequences are disclosed in European Patent Application Publication No. 0 401 979, published Dec. 12, 1990, of Mycogen Corporation. One of the disclosed sequences (PS82A2) appears to encode a novel gene, the other sequence (PS82RR) is highly related to the novel sequence cry 81IA2 disclosed in European Patent Application Publication No. 0 405 810.
Five novel cry sequences are disclosed in European Patent Application Publication No. 0 462 721, published Dec. 27, 1991, of Mycogen Corporation. These Cry proteins are reported to be nematocidal.